Less than 4% Si steels are classified into grain oriented Si steels and non-oriented steels in accordance with the producing practices, and are processed to laminated iron cores or coiled iron cores for electromagnetic induction devices, or magnetic shielding cases.
Recently, from the standpoint of saving natural sources and energy, small sizes and high efficiency electromagnetic or electronic parts have been demanded, and soft magnetic properties, especially in Si steel sheets having excellent iron loss properties, have been also required. It is known that soft magnetic properties of Si steel sheets are improved with increasing additions of Si and exhibit the maximum permeability at about 6.5 wt %, and since natural electric resistance is high, the iron loss is made small.
In this kind of steel sheets, if Si content is more than 4.0 wt %, workability is abruptly worsened, and therefore it has been impossible to produce high Si steel sheets in industrial scales by the rolling process.
Many patents and literatures teach the rolling of high Si steels. Those almost always refer to steel materials of below 4.0 wt %, or if some of them describe Si content of above 4.0 wt %, such descriptions seem to be analogized from materials of about 3 wt %. Upon the inventors' many experiments and investigations made on materials of around 6.5 wt %, it was found that high Si steel sheets as 6.5 wt % could not be produced by the above taught procedures of the prior art.
Productions of Si steel sheets are disclosed, for example, in Laid-Open Japanese Patent Application Nos. 29496/76, 36968/82 or 181822/83, but those deal with materials of lower than 4.0 wt % and could not be applied to Si steels of around 6.5 wt % because workability is abruptly dropped with increasing of Si content.
It is known that the rollings are carried out on brittle materials or materials with high deformation resistance not by the cold working but by increasing the temperature. The greatest problem with producing high Si thin steel sheets is how to avoid troubles caused by crackings in each of the producing steps and how accomplish a stable total process. Satisfactory results could not be obtained by merely heightening the temperature.
The inventors developed studies about productions by rolling high Si thin steel sheets of more than 4.0 wt % Si content. In the course of their studies, it was found that the productions by rolling had the following problems.
(1) During cooling while transferring the steel ingot, slab, or continuously cast slab, thermal stress cracks are generated due to difference in temperature between the surface and the interior. PA0 (2) Since the processability is largely changed by the processing degree of the material, i.e., the structure, rolling cracks would be generated unless the rolling temperatures were selected properly. PA0 (3) Unless the coiling temperatures were selected properly, the coil would be broken when the temperature is low, and when the temperature is high deformation in a next rolling would be worsened considerably by recrystallization of the coiled steel. PA0 (a) introducing a solidified ingot or a continuously cast piece into a slab heating furnace until the lowest-temperature part thereof becomes not less than 600.degree. C., heating it at temperature of not more than 1250.degree. C. therein and rolling the slab; or PA0 (b) directly transferring a solidified ingot or a continuously cast piece into a slabbing process while the lowest-temperature part thereof is becomes not less than 600.degree. C.; after rolling the slab at temperature of not less than 600.degree. C., PA0 (a) introducing a solidified cast piece into a roll heating furnace until the lowest-temperature part thereof becomes not less than 600.degree. C., and sending the heated piece to a hot rolling process; or PA0 (b) directly transferring the solidified cast piece to the hot rolling process while the lowest-temperature part thereof is not less than 600.degree. C.
Through further studies in reference to the above problems, it was found that the problems (1) to (3) were improved and that high Si steel sheets could be produced stably without inviting problems from making the molten steel into the final thickness.